Things happen in life that requires dropping everything, but these are life and death situations - major misfortune that any decent employer would totally understand about an employee attending to.
If, in the very improbable event, you find yourself in one of these conditions, all you can do is communicate openly and honestly with your prospective employer, apologize and explain the situation, and hope that you will not have had too much of a negative effect on your chances of employment.
The truth though is that no matter how good and valid a reason you have for rescheduling, you will not look as professional, prepared and reliable as you would have done if you had turned up at the time you had first agreed.
You must remember that your upcoming employer is not stupid and naive, so do not go down the route of making up a reason to reschedule. It is very improbable that you will be believed and nobody likes a liar. All the excuses you can think of will have been used and seen through uncountable times before.
Life is tough though, and things can go wrong on the day - ridiculous things, like your alarm not going off, or getting stuck in traffic, or be wrong about the travelling time to the interview. Of course it is very easy to point out that you should not have allowed any of these to happen, but if things do go wrong, all you can do is go into damage control. Get straight in contact with the prospective employer, if possible before the interview time, and hope that they will reschedule. Do not be amazed though, if you do not get the response you had hoped for.
You really have to see at it from an employer's point of view. Would you want to employ someone, paying a salary and giving errands that are important to your business, if they did not even show the clerical skills, dependency and good attitude to stick to a job interview on time?